This invention relates to active matrix electroluminescent display devices comprising a matrix array of electroluminescent display elements each of which has an associated switching means for controlling the current through the display element, in accordance with an applied drive signal.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. As for the display elements organic thin film electroluminescent elements and light-emitting diodes (LEDs), comprising traditional III-V semiconductor compounds, have been used. In the main, such display devices have been of the passive type in which the electroluminescent display elements are connected between intersecting sets of row and column address lines and addressed in multiplexed fashion. Recent developments in (organic) polymer electroluminescent materials have demonstrated their ability to be used practically for video display purposes and the like. Electroluminescent elements using such materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of (anode and cathode) electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. An example of such is described in an article by D. Braun and A. J. Heeger in Applied Physics Letters 58 (18) p.p. 1982-1984 (May 6th 1991). By suitable choice of the conjugated polymer chain and side chains, it is possible to adjust the bandgap, electron affinity and the ionisation potential of the polymer. An active layer of such a material can be fabricated using a CVD process or simply by a spin-coating technique using a solution of a soluble conjugated polymer. Through these processes, LEDs and displays with large light-emitting surfaces can be produced.
Organic electroluminescent materials offer advantages in that they are very efficient and require relatively low (DC) drive voltages. Moreover, in contrast to conventional LCDs, no backlight is required. In a simple matrix display device, the material is provided between sets of row and column address conductors at their intersections thereby forming a row and column array of electroluminescent display elements. By virtue of the diode-like I-V characteristic of the organic electroluminescent display elements, each element is capable of providing both a display and a switching function enabling multiplexed drive operation. However, when driving this simple matrix arrangement on a conventional row at a time scanning basis each display element is driven to emit light for only a small fraction of the overall field time, corresponding to a row address period. In the case of an array having N rows for example, each display element can emit light for a period equal to f/N at most where f is the field period. In order then to obtain a desired mean brightness from the display, it is necessary that the peak brightness produced by each element must be at least N times the required mean brightness and the peak display element current will be at least N times the mean current. The resulting high peak currents cause problems, notably with the more rapid degradation of the display element lifetime and with voltage drops caused along the row address conductors.
One solution to these problems is to incorporate the display elements into an active matrix whereby each display element has an associated switch means which is operable to supply a drive current to the display element so as to maintain its light output for a significantly longer period than the row address period. Thus, for example, each display element circuit is loaded with an analogue (display data) drive signal once per field period in a respective row address period which drive signal is stored and is effective to maintain a required drive current through the display element for a field period until the row of display elements concerned is next addressed. This reduces the peak brightness and the peak current required by each display element by a factor of approximately N for a display with N rows. An example of such an active matrix addressed electroluminescent display device is described in EP-A-0717446. The conventional kind of active matrix circuitry used in LCDs cannot be used with electroluminescent display elements as such display elements need to continuously pass current in order to generate light whereas the LC display elements are capacitive and therefore take virtually no current and allow the drive signal voltage to be stored in the capacitance for the whole field period. In the aforementioned publication, each switch means comprises two TFTs (thin film transistors) and a storage capacitor. The anode of the display element is connected to the drain of the second TFT and the first TFT is connected to the gate of the second TFT which is connected also to one side of the capacitor. During a row address period, the first TFT is turned on by means of a row selection (gating) signal and a drive (data) signal is transferred via this TFT to the capacitor. After the removal of the selection signal the first TFT turns off and the voltage stored on the capacitor, constituting a gate voltage for the second TFT, is responsible for operation of the second TFT which is arranged to deliver electrical current to the display element. The gate of the first TFT is connected to a gate line (row conductor) common to all display elements in the same row and the source of the first TFT is connected to a source line (column conductor) common to all display elements in the same column. The drain and source electrodes of the second TFT are connected to the anode of the display element and a ground line which extends parallel to the source line and is common to all display elements in the same column. The other side of the capacitor is also connected to this ground line. The active matrix structure is fabricated on a suitable transparent, insulating, support, for example of glass, using thin film deposition and process technology similar to that used in the manufacture of AMLCDs.
With this arrangement, the drive current for the light-emitting diode display element is determined by a voltage applied to the gate of the second TFT. This current therefore depends strongly on the characteristics of that TFT. Variations in threshold voltage, mobility and dimensions of the TFT will produce unwanted variations in the display element current, and hence its light output. Such variations in the second TFTs associated with display elements over the area of the array, or between different arrays, due, for example, to manufacturing processes, lead to non-uniformity of light outputs from the display elements.
It is an object of the present invention to provide an improved active matrix electroluminescent display device.
It is another object of the present invention to provide a display element circuit for an active matrix electroluminescent display device which reduces the effect of variations in the transistor characteristics on the light output of the display elements and hence improves the uniformity of the display.
This objective is achieved in the present invention by using a current mirror circuit for the switching means in which the same transistor is used to both sense and later produce the required drive current for the display element. This allows all variations in transistor characteristics to be compensated.
According to the present invention, there is provided an active matrix electroluminescent display device of the kind described in the opening paragraph, in which the switching means comprises a drive transistor whose first current-carrying terminal is connected to a first supply line, whose second current-carrying terminal is connected via the display element to a second supply line and whose gate is connected to its first current-carrying terminal via a capacitance, which is characterized in that the second current-carrying terminal of the drive transistor is connected to an input terminal for the drive signal and in that a switch device is connected between the second current-carrying terminal and the gate of the transistor which is operable during the application of a drive signal so as to store on the capacitance a gate voltage determined by the drive signal.
The arrangement of the switching means is such that it operates effectively in the manner of a single transistor current mirror circuit wherein the same transistor performs current sampling and current output functions. When the switch device is closed the transistor is diode connected and the input drive signal determines a current flow through the transistor and a consequential gate voltage which is stored on the capacitance. After the switch device opens, the transistor acts as a current source for the display element with the gate voltage determining the current level through the display element, and hence its brightness, which level is thereafter maintained, according to the set value, for example until the display element is next addressed. Thus, in a first operating phase, in effect a display element addressing period, an input current is sampled and the transistor gate voltage set accordingly and in a subsequent output phase the transistor operates to draw a current through the display element corresponding to the sampled current. Because in this arrangement the same transistor is used both to sample the input current during the sampling phase and to generate the drive current for the display element during the output phase the display element current is not dependent on the threshold voltage, the mobility, or the exact geometry of the transistor. The aforementioned problems of non-uniformity of light outputs from the display elements over the array is thus overcome.
Preferably, the display elements are arranged in rows and columns, and the switch devices of the switching means for a row of display elements are connected to a respective, common, row address conductor via which a selection (scan) signal for operating the switch devices in that row is supplied, and each row address conductor is arranged to receive a selection signal in turn, whereby the rows of display elements are addressed one at a time in sequence. The drive signals (display data) for the display elements in a column are preferably supplied via a respective column address conductor common to the display elements in the column, there being a further switch device connected between the input terminal of the switching means of a display element and its associated column address conductor which is operable to transfer a drive signal on the column address conductor to the input terminal when the first-mentioned switch device is closed. To this end, the further switch device is preferably connected to the same row address conductor as the first-mentioned switch device and operable simultaneously with that switch device by the selection signal applied to the row address conductor. During the time when the display element is not being addressed, i.e. the output phase, this further switching device serves to isolate the input terminal from the column address conductor.
Preferably the first supply line is shared by all display elements in the same row or column. A respective supply line may be provided for each row or column of display elements. Alternatively, a supply line could effectively be shared by all the display elements in the array using, for example, lines extending in the column or row direction and connected together at their ends or by using lines extending in both the column and the row directions and connected together in the form of a grid. The approach selected will depend on the technological details for a given design and fabrication process.
For simplicity, a first supply line which is associated, and shared by, a row of display elements may comprise the row address conductor associated with a different, preferably adjacent, row of display elements via which a selection signal is applied to the switch devices of the switching means of that different row.
The switch devices preferably also comprise transistors and all transistors may conveniently be formed as TFTs on a substrate of glass or other insulating material together with the address conductors using standard thin film deposition and patterning processes as used in the field of active matrix display devices and other large area electronic devices. It is envisaged however, that, the active matrix circuitry of the device may be fabricated using IC technology with a semiconductor substrate.
In order to prevent current flow through the display element during the sampling phase another switch device may be connected between the second current-carrying terminal of the drive transistor and the display element which is operable to isolate the display element from the drive transistor during the sampling phase. This switch device may similarly comprise a switching transistor but of opposite conductivity type to the transistors constituting the other switching devices so that, with its gate connected to the same row address conductor, it operates in complementary fashion. Thus, this transistor may comprise a p-channel device while the first-mentioned and further transistors comprise n-channel devices. Of course, by reversing the polarity of the display element and the polarity of the waveform applied to the row address conductors, the above transistor types can be reversed.
The need for such a complementary-operating switch device can be avoided. In a preferred embodiment a pulse signal is arranged to be applied to the first supply line, and thus the first current-carrying electrode of the drive transistor, during the sampling phase which reverse biases the display element, thereby preventing current flow through the display element and ensuring that the drain current through the drive transistor corresponds to the input signal current and that the appropriate gate-source voltage is sampled on the capacitance. In the case of the first supply line comprising a row address conductor associated with an adjacent row of display elements, this pulse is provided separate to the selection signal on that row address conductor and coincident in time with the selection signal on the row address conductor associated with the display element concerned. The amplitude of the pulse required is less than that of the selection signal. Besides reducing the total number of transistors required, the avoidance of a switching transistor connected between the second current-carrying terminal of the driving transistor and the display element simplifies fabrication as the transistors then needed are all of the same polarity type.